Timing based LNA gain adjustment in an RF receiver to compensate for intermodulation interference

Abstract

A Radio Frequency (RF) receiver includes a low noise amplifier (LNA) and a mixer coupled to the output of the LNA. The gain of the LNA is adjusted to maximize signal-to-noise ratio of the mixer and to force the mixer to operate well within its linear region when an intermodulation interference component is present. The RF receiver includes a first received signal strength indicator (RSSI_A) coupled to the output of the mixer that measures the strength of the wideband signal at that point. A second received signal strength indicator (RSSI_B) couples after the BPF and measures the strength of the narrowband signal. The LNA gain is set based upon these signal strengths. LNA gain is determined during a guard period preceding an intended time slot of a current frame and during a guard period following an intended time slot of a prior frame. The lesser of these two LNA gains is used for the intended time slot of the current frame.

1. A method for adjusting the gain of a Low Noise Amplifier (LNA) in a Radio Frequency (RF) receiver of a wireless device, the method comprising:
 determining a LNA gain setting during a guard period immediately following an intended time slot of a prior data frame;  
 determining a LNA gain setting during a guard period immediately preceding an intended time slot of a current data frame; and  
 setting the gain of the LNA for the intended time slot of the current data frame to the lesser of the LNA gain setting determined during the guard period immediately following the intended time slot of the prior data frame and the LNA gain setting determined during the guard period preceding the intended time slot of the current data frame.  
 
   
   
     2. The method of  claim 1 , wherein the prior data frame immediately precedes the current data frame. 
   
   
     3. The method of  claim 1 , wherein:
 no intermodulation interference exists in the receiver during the guard period immediately preceding the intended time slot of the current data frame; and  
 an intermodulation interference exists in the receiver during a guard period immediately following an intended time slot of a prior data frame.  
 
   
   
     4. The method of  claim 1 , wherein:
 multiple time slots of each of the data frames are intended for the wireless device;  
 a first guard period precedes a first one of the multiple time slots; and  
 a second guard period immediately follows a last one of the multiple time slots.  
 
   
   
     5. The method of  claim 1 , wherein determining a LNA gain setting during a guard period comprises:
 measuring wideband signal strength on an output side of a mixer of the RF receiver (RSSI_A);  
 measuring narrowband signal strength on an output side of the mixer of the RF receiver (RSSI_B);  
 determining that intermodulation interference exists; and  
 determining a gain setting of the LNA based upon the existence of the intermodulation interference to cause the mixer to operate linearly.  
 
   
   
     6. The method of  claim 5 , wherein determining that intermodulation interference exists comprises:
 adjusting the LNA gain by a gain adjustment step;  
 measuring again narrowband signal strength after the BPF (RSSI_B′); and  
 determining that the difference between RSSI_B and RSSI_B′ is not linearly related to the gain adjustment step, such determination indicating the presence of intermodulation interference.  
 
   
   
     7. The method of  claim 5 , wherein determining that the difference between RSSI_B and RSSI_B′ is not linearly related to the gain adjustment step is determined by comparing the difference between RSSI_B and RSSI_B′ to a threshold that is based upon the LNA gain. 
   
   
     8. The method of  claim 5 , further comprising not adjusting the LNA gain when RSSI_A is less than a predefined threshold (THRS_B). 
   
   
     9. The method of  claim 5 , wherein determining that intermodulation interference exists is performed only when RSSI_A is greater than a predefined threshold (THRS_B). 
   
   
     10. The method of  claim 5 , further comprising reducing the LNA gain when RSSI_A exceeds a predefined threshold (THRS_A). 
   
   
     11. A method for adjusting the gain of a Low Noise Amplifier (LNA) in an RF unit that includes the LNA, a mixer coupled to the output of the LNA, a Band Pass Filter (BPF) coupled to the output of the mixer, a first received signal strength indicator (RSSI_A) coupled to measure the wideband signal strength at the output of the mixer, and a second received signal strength indicator (RSSI B) coupled to measure the narrowband signal strength after the BPF, the method comprising:
 determining a LNA gain setting during a guard period immediately following an intended time slot of a prior data frame;  
 determining a LNA gain setting during a guard period immediately preceding an intended time slot of a current data frame;  
 setting the gain of the LNA for the intended time slot of the current data frame to the lesser of the LNA gain setting determined during the guard period immediately following the intended time slot of the prior data frame and the LNA gain setting determined during the guard period preceding the intended time slot of the current data frame; and  
 wherein determining a LNA gain setting comprises: 
 when intermodulation interference does not exist, selecting a relatively lower LNA gain setting; and  
 when intermodulation interference exists, selecting a relatively higher LNA gain setting.  
 
 
   
   
     12. The method of  claim 11 , wherein the prior data frame immediately precedes the current data frame. 
   
   
     13. The method of  claim 11 , wherein:
 no intermodulation interference exists in the receiver during the guard period immediately preceding the intended time slot of the current data frame; and  
 an intermodulation interference exists in the receiver during a guard period immediately following an intended time slot of a prior data frame.  
 
   
   
     14. The method of  claim 11 , wherein:
 multiple time slots of each of the data frames are intended for the wireless device;  
 a first guard period precedes a first one of the multiple time slots; and  
 a second guard period immediately follows a last one of the multiple time slots.  
 
   
   
     15. The method of  claim 11 , wherein determining a LNA gain setting during a guard period comprises:
 measuring wideband signal strength at the output of the mixer (RSSI_A);  
 measuring narrowband signal strength after the BPF (RSSI_B);  
 adjusting a gain (the LNA gain) of the LNA;  
 determining that intermodulation interference exists by: 
 adjusting the LNA gain by a gain adjustment step;  
 measuring again a signal strength after the BPF (RSSI_B′); and  
 determining that the difference between RSSI_B and RSSI_B′ is not linearly related to the gain adjustment step; and  
 
 adjusting the gain of the LNA based upon the existence of the intermodulation interference to cause the mixer to operate linearly.  
 
   
   
     16. The method of  claim 15 , wherein determining that the difference between RSSI_B and RSSI_B′ is not linearly related to the gain adjustment step is determined by comparing the difference between RSSI_B and RSSI_B′ to a threshold that is based upon the LNA gain. 
   
   
     17. The method of  claim 15 , further comprising not adjusting the LNA gain when RSSI_A is less than a predefined threshold (THRS_B). 
   
   
     18. The method of  claim 15 , wherein determining that intermodulation interference exists is performed only when RSSI_A is greater than a predefined threshold (THRS_B). 
   
   
     19. The method of  claim 15 , further comprising reducing the LNA gain when RSSI_A exceeds a predefined threshold (THRS_A). 
   
   
     20. A Radio Frequency unit comprising:
 a transmit/receive block that couples to an antenna;  
 transmit circuitry that receives Intermediate Frequency (IF) transmit data from a host device, that converts the IF transmit data to an RF transmit signal and that couples the RF transmit signal to the transmit/receive block for transmission;  
 a Low Noise Amplifier (LNA) operably coupled to the transmit/receive block to receive an RF receive signal;  
 a mixer coupled to receive an output of the LNA;  
 a Band Pass Filter (BPF) coupled to an output of the mixer;  
 a first received signal strength indicator coupled to the output of the mixer that indicates a wideband received signal strength (RSSI_A);  
 a second received signal strength indicator coupled after the BPF that indicates a narrowband received signal strength (RSSI_B); and  
 a LNA gain adjustment block that operates to: 
 determine a LNA gain setting during a guard period immediately following an intended time slot of a prior data frame;  
 determine a LNA gain setting during a guard period immediately preceding an intended time slot of a current data frame; and  
 set the gain of the LNA for the intended time slot of the current data frame to the lesser of the LNA gain setting determined during the guard period immediately following the intended time slot of the prior data frame and the LNA gain setting determined during the guard period preceding the intended time slot of the current data frame.  
 
 
   
   
     21. The Radio Frequency unit of  claim 20 , wherein the LNA gain adjustment block:
 determines that intermodulation interference exists; and  
 adjusts the gain of the LNA based upon the existence of the intermodulation interference to cause the mixer to operate linearly.  
 
   
   
     22. The Radio Frequency unit of  claim 20 , wherein in determining that intermodulation interference exists comprises, the LNA gain adjustment block:
 records RSSI_B;  
 adjusts the LNA gain by a gain adjustment step; ID records a new RSSI_B (RSSI_B′); and  
 determines that the difference between RSSI_B and RSSI_B′ is not linearly related to the gain adjustment step, such determination indicating the presence of intermodulation interference.  
 
   
   
     23. The Radio Frequency unit of  claim 22 , wherein in determining that intermodulation interference exists, the LNA gain adjustment block compares the difference between RSSI_B and RSSI_B′ to a threshold that is based upon the LNA gain. 
   
   
     24. The Radio Frequency unit of  claim 22 , wherein the LNA gain adjustment block does not adjust the LNA gain when RSSI_A is less than a predefined threshold (THRS_B). 
   
   
     25. The Radio Frequency unit of  claim 22 , wherein the LNA gain adjustment block reduces the LNA gain when RSSI_A exceeds a predefined threshold (THRS_A).